Optimizing focus point for optical disc

ABSTRACT

An apparatus includes a first circuit and a second circuit. The first circuit may be configured to generate status signals and an error signal in response to accessing a storage medium. The error signal provides a first value based on an accuracy of accessing the storage medium. The second circuit may be configured to offset the first value of the error signal to a second value to increase the accuracy of accessing said storage medium. The status signals include one or more of a data signal and a differential signal. In a first mode, an offset signal is generated in response to the data signal. In a second mode, the offset signal is generated in response to the differential signal.

This is a continuation of U.S. Ser. No. 11/450,846, filed Jun. 9, 2006,now U.S. Pat. No. 7,633,840, which is incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to optical storage generally and, moreparticularly, to a method and/or apparatus for optimizing a focus pointfor optical disc.

BACKGROUND OF THE INVENTION

In a conventional optical disc system, to sense the position of thelaser beam in relation to the track on the disc, a main laser beamcreates a reflection from the disc. The reflection from the main laserbeam is picked up by 4 photo-diode sensors within a photo-diode sensorassembly. FIG. 1 is a conceptual diagram illustrating how such aphoto-diode sensor assembly 10 is laid out in relation to the trackdirection. The outputs of the 4 photo-diodes within the photo-diodeassembly 10 (when the laser beam is focused on the disc) are shown assignals A, B, C and D, respectively.

Referring to FIG. 2, a conventional optical disc system 20 is shown. Afocus actuator (not shown) will keep a laser beam 22 focused on asurface of the disc 23 by adjusting the vertical position of a lens 24.A focus controller (not shown) controls the focus actuator. To controlthe focus actuator to keep the laser beam 22 focused on the surface ofthe disc 23, a signal focus error (FE) is controlled to zero and asignal beam strength (BS) is controlled to a high value. The signal FEprovides information related to the vertical position of the lens 24.The signal BS provides information related to the strength of the laserbeam 22. Due to the alignment of the photo-diode sensor 10 when thesignal FE is controlled to zero, the focus point of the laser beam 22may not be optimally positioned on the surface of the disc 23. It isnecessary to adjust the signal FE slightly off of a zero level in orderto optimally establish the focus point of the laser beam 22 on thesurface of the disc 23. Conventional methods fail to provide an optimalfocus point of the laser beam on the surface of the disc when the signalFE is slightly off a zero level (or at an optimized focus offset level).Since conventional methods fail to provide an optimal focus point of thelaser beam on the disc, the overall quality of reading and writing datafrom and to the disc will be decreased.

It would be desirable to provide a method and/or apparatus to optimizethe focus point for an optical disc.

SUMMARY OF THE INVENTION

The present invention concerns an apparatus including a first circuitand a second circuit. The first circuit may be configured to generatestatus signals and an error signal in response to accessing a storagemedium. The error signal provides a first value based on an accuracy ofaccessing the storage medium. The second circuit may be configured tooffset the first value of the error signal to a second value to increasethe accuracy of accessing said storage medium. The status signalsinclude one or more of a data signal and a differential signal. In afirst mode, an offset signal is generated in response to the datasignal. In a second mode, the offset signal is generated in response tothe differential signal.

The objects, features and advantages of the present invention includeproviding a method and/or apparatus for optimizing the focus point foran optical disc that may (i) provide for a reliable method of optimizingthe focus point of a laser beam, (ii) optimize the focus point of alaser beam to read and write data on an optical disc, (iii) increasequality in the read/write process, (iv) be inexpensive to implementand/or (v) be easy to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 illustrates a photo-diode sensor distribution;

FIG. 2 illustrates a lens and a lens housing in relation to a positionof a laser beam;

FIG. 3 is a diagram of a system incorporating the present invention;

FIG. 4 is a more detailed diagram of the data circuit in the context ofthe present invention;

FIG. 5 is a diagram illustrating a data signal creation circuit;

FIG. 6 is a diagram illustrating a main beam push-pull signal creationcircuit;

FIG. 7 is a diagram illustrating a focus error signal creation circuit;

FIG. 8 is a flow diagram for finding the optimal focus point to readdata from an optical disc;

FIG. 9 is a diagram illustrating the peak-to-peak value of the signal DSas the focus offset is changed;

FIG. 10 is a flow diagram for finding the optimal focus point to recorddata on an optical disc; and

FIG. 11 is a diagram illustrating the peak-to-peak value of the mainbeam peak-to-peak signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, a block diagram of a system 100 is shown inaccordance with a preferred embodiment of the present invention. Thesystem 100 generally comprises an optical pick-up unit (OPU) 102, anoptical disc 104, a number of disc tracks 105 a-105 n, a block (orcircuit) 106, a block (or circuit) 108, a block (or circuit) 110, ablock (or circuit) 112, a block (or circuit) 114 and a laser source 120.The circuit 106 may be implemented as data circuit. The circuit 108 maybe implemented as a focus error creation circuit. The circuit 110 may beimplemented as a focus offset circuit. The circuit 112 may beimplemented as an adder circuit. The circuit 114 may be implemented as afocus controller. The circuits 110, 112 and 114 may be implemented viahardware or software. In one example, the circuits 110, 112 and 114 maybe implemented as hardware circuits. Depending on the particularhardware design, the circuits 110, 112 and 114 may be analog and/ordigital circuits. In one example, the circuits 110, 112 and 114 may eachbe implemented as a software block (e.g., a digital signal processingsoftware structure) without the use of a hardware circuit. Theimplementation of the circuits 110, 112 and 114 as software or hardwaremay be varied to meet the design criteria of a particularimplementation. The OPU 102 generally comprises a photo-diode sensor122, a lens 126, a sled housing 128 and a focus actuator 130.

The photo-diode sensor 122 may present any combination of signals A, B,C and D on a signal (e.g., PD_1) to the data circuit 106. Thephoto-diode sensor 122 may present any combination of signals A, B, C,and D on a signal (e.g., PD_2) to the data circuit 106. The photo-diodesensor 122 may present any combination of signals A, B, C and D on asignal (e.g., PD_3) to the focus error creation circuit 108. The datacircuit 106 may present a data signal (e.g., DS) to the focus offsetcircuit 110. The data circuit 106 may present a signal (e.g., MBPP) tothe focus offset circuit 110. The focus offset circuit 110 may present asignal (e.g., OFFSET) to the adder 112. The focus error creation circuit108 may present the signal FE to the adder circuit 112. The addercircuit 112 may present a signal (e.g., FE_AFTER_OFFSET) to the focuscontroller 114. The focus controller 114 may control the verticalposition of the lens 126 with a signal (e.g., CTRL).

In a first state, the data circuit 106 may present the signal DS to thefocus offset circuit 110 when reading data from the disc 104. The focusoffset circuit 110 may generate the signal OFFSET in response to thesignal DS. In a second state, the data circuit 106 may present thesignal MBPP to the focus offset circuit 110 when writing or rewriting(recording) data to the disc 104. The focus offset circuit 110 maygenerate the signal OFFSET in response to the signal MBPP. The signal FEmay be at zero (or at a first value) when the focus controller 114 keepsthe laser beam 124 focused on the surface of the disc 104. The focusoffset circuit 110 may offset the signal FE to a level off of zero (orto a second value) to ensure that the focus point of the laser beam 124is at an optimal position. The focus offset circuit 110 may adjust (orincrease) the focus point of the laser beam 124 when (i) reading datafrom any one of a particular tracks 105 a-105 n and (ii) writing data toany one of a particular tracks 105 a-105 n.

Referring to FIG. 4, a more detailed diagram of the data circuit 106 inthe context of the present invention is shown. The data circuit 106generally comprises a block (or circuit) 116 and a block (or circuit)118. The circuit 116 may be implemented as a data signal creationcircuit. The circuit 118 may be implemented as a push-pull signalcreation circuit. The focus offset circuit 110 may generate the signalOFFSET when (i) data is read from any one of a particular tracks 105a-105 n and (ii) data is written to any one of a particular tracks 105a-105 n.

In the first state, when reading data from the disc 104, the data signalcreation circuit 116 may generate the signal DS. The signal DS may be aradio frequency signal. The signal DS may be generated from data on theoptical disc 104. The focus offset circuit 110 may generate the signalOFFSET based on the value of the signal DS. The signal FE may be zerowhen the focus actuator 130 adjusts the focus point of the laser beam124 on the surface of the disc 104 (e.g., the focus actuator 130 adjuststhe vertical position of the lens 126 with respect to the laser beam124). The focus actuator 130 may be positioned within the sled housing128. The focus actuator 130 may be implemented as a voice coil motor.The focus actuator 130 may act as a spring and move the lens 126vertically. The focus actuator 130 may adjust the vertical position ofthe lens 126 by controlling the amount of current that flows through acoil when in the presence of a magnetic field. The focus actuator 130may be integrated as a hardware device within the OPU 102.

The adder circuit 112 may add the signal OFFSET to the signal FE togenerate the signal FE_AFTER_OFFSET. In general, the system 100 may notobtain an optimal focus point of the laser beam 124 when the signal FEis equal to zero. The focus controller 114 may control the focusactuator 130 such that the signal FE_AFTER_OFFSET is always at zero.When the signal FE_AFTER_OFFSET is zero, an optimal focus point of thelaser beam 124 may be achieved. If the signal OFFSET is zero, such acondition may illustrate that the signal FE is at zero (e.g., the signalFE_AFTER_OFFSET may be set to zero) and an optimal focus point of thelaser beam 124 has been achieved. If the signal OFFSET is not zero, thefocus actuator 130 (via control of the focus controller 114) may adjustthe vertical position of the lens 124 such that the focus error creationcircuit 108 may generate the signal FE to be at a reverse level (orvalue) of the signal OFFSET. The sum of the signal OFFSET and the signalFE (which is at a reverse level of the signal OFFSET) may ensure thatthe signal FE_AFTER_OFFSET is set to zero. In general, the signal OFFSETmay control the focus actuator 130 to adjust the focus point of thelaser beam 124 in order to achieve the optimal focus point. While in thefirst state, the laser beam 124 may be (i) focused on the surface of thedisc 104 and (ii) controlled (by a tracking actuator (not shown)) tostay on any one of a particular tracks 105 a-105 n.

In the second state, when the disc 104 is recordable (e.g., data iswritten to the disc 104), the beam push-pull signal creation circuit 118may generate the signal MBPP. The signal MBPP may be a low frequencysignal. The focus offset circuit 110 may generate the signal OFFSETbased on the value of the signal MBPP. The signal FE may be zero whenthe focus actuator 132 adjusts the focus of the laser beam 124 on thesurface of the disc 104. The adder circuit 112 may add the signal OFFSETto the signal FE to generate the signal FE_AFTER_OFFSET. In general, thesystem 100 may not obtain an optimal focus point of the laser beam 124when the signal FE is equal to zero. The focus controller 114 maycontrol the focus actuator 130 such that the signal FE_AFTER_OFFSET isalways at zero. When the signal FE_AFTER_OFFSET is zero, an optimalfocus point of the laser beam 124 may be achieved. If the signal OFFSETis zero, such a condition may illustrate that the signal FE is at zero(e.g., the signal FE_AFTER_OFFSET may be set to zero) and an optimalfocus point of the laser beam 124 has been achieved. If the signalOFFSET is not zero, the focus actuator 130 (via the control of the focuscontroller 114) may adjust the vertical position of the lens 124 suchthat the focus error creation circuit 108 may generate the signal FE tobe at a reverse level of the signal OFFSET. The sum of the signal OFFSETand the signal FE (which is at a reverse level of the signal OFFSET) mayensure that the signal FE_AFTER_OFFSET is set to zero. While in thesecond state, the laser beam 124 may be focused on the surface of thedisc 104. The laser beam 124 may not be controlled to stay on any one ofa particular tracks 105 a-105 n.

Referring to FIG. 5, a more detailed diagram of the data signal creationcircuit 116 is shown. The data signal creation circuit 116 generallycomprises a circuit 130, a circuit 132, a circuit 134, and a circuit136. The circuit 130, the circuit 132 and the circuit 136 may beimplemented as summing circuits. The circuit 136 may be implemented as aradio frequency filter. The circuit 130 may (i) receive the signal B andthe signal D and (ii) present a signal equal to B+D. Similarly, thecircuit 132 may (i) receive the signal A and the signal C and (ii)present an output signal equal to A+C. The summing circuit 134 may (i)receive the signal A+C and the signal B+D and (ii) present a signalequal to (A+C)+(B+D). The signal (A+C)+(B+D) may be presented to theradio frequency filter 136. The radio frequency filter 136 may generatethe signal DS. The signal DS or (radio frequency signal) may be a highfrequency signal. The radio frequency filter 136 may be implemented as ahigh pass filter that is combined with one or more low pass filters. Thedata signal creation circuit 116 may generate the signal DS from data onthe optical disc 104 as the laser beam 124 is focused on the surface ofthe disc 104 while the disc 104 is spinning.

Referring to FIG. 6, a more detailed diagram of the beam push pullcreation circuit 118 is shown. The beam push pull creation circuit 118generally comprises a circuit 150, a circuit 152, a circuit 154, and acircuit 156. The circuit 150 and the circuit 152 may be implemented assumming circuits. The circuit 154 may be implemented as a differentialcircuit (e.g., a comparator, etc.). The circuit 156 may be implementedas a low pass filter. The circuit 150 may (i) receive the signal A andthe signal D from the photo-diode sensor 122 and (ii) present a signalequal to A+D. Similarly, the circuit 152 may (i) receive the signal Band the signal C from the photo-diode sensor 122 and (ii) present anoutput signal equal to B+C. The differential circuit 154 may (i) receivethe signal A+D and the signal B+C and (ii) present a signal equal to(A+D)−(B+D). The signal (A+D)−(B+C) may be presented to the low passfilter 156, which may generate the signal MBPP. The signal MBPP may be alow frequency signal. The signal MBPP may be generated when (i) thelaser beam 124 is focused on the surface of the disc 104 and (ii) thedisc 104 is spinning. The disc 104 may (i) be independent of data (e.g.,a blank disc) or (ii) include data (e.g., a rewrittable disc). Due todifferences between the read and write processes, the optimal focuspoint of the laser beam 124 may be different.

Referring to FIG. 7, a more detailed diagram of the focus error creationcircuit 108 is shown. The focus error creation circuit 108 generallycomprises a circuit 170, a circuit 172, a circuit 174 and a circuit 176.The circuit 170 and the circuit 172 may be implemented as summingcircuits. The circuit 174 may be implemented as a differential circuit(e.g., a comparator, etc.). The circuit 176 may be implemented as a lowpass filter. The circuit 170 may (i) receive the signal B and the signalD and (ii) present a signal equal to B+D. Similarly, the circuit 172 may(i) receive the signal A and the signal C and (ii) present an outputsignal equal to A+C. The differential circuit 174 may (i) receive thesignal A+C and the signal B+D and (ii) present a signal equal to(A+C)−(B+D). The signal (A+C)−(B+D) may be presented to the low passfilter 176, which generates the signal FE.

Referring to FIG. 8, a method 200 for optimizing the focus point to readdata from an optical disc is shown. The method 200 generally comprises astate (or step) 202, state (or step) 204, a state (or step) 206, a state(or step) 208, a state (or step) 210, a state (or step) 212, a state (orstep) 214, a state (or step) 216 and a state (or step) 218. The state202 may be a start state. The state 218 may be an end state.

The state 204 may control the lens 126 to keep the laser beam 124focused on the surface of the disc 104. When the lens 126 is controlledby the focus controller 114 so that the laser beam 124 is focused on thedisc 104, the laser beam 124 may be controlled to lock to any one of aparticular tracks of data 105 a-105 n on the disc 104. The state 206 mayapply a predetermined amount of constant offset in the signal OFFSET.The predetermined amount of offset in the signal OFFSET may be added tothe signal FE to generate the signal FE_AFTER_OFFSET. The state 208 maymeasure the peak-to-peak value of signal DS as the signal OFFSET isadded to the signal FE to generate the signal FE_AFTER_OFFSET.

The state 210 may gradually adjust (by increasing or decreasing) thesignal OFFSET by a fixed predetermined amount. The state 212 may repeatstates 206, 208, and 210 a number of times (e.g., N). The step 208 maycollect Xi and Yi values (where i=1, 2, . . . , N). The Yi value maycorrespond to the measured peak-to-peak values of the signal DS when thestates 206 and 208 are repeated N-times. The Xi value may correspond tothe adjusted values of the signal OFFSET when the state 210 is repeatedN times.

The state 214 may find a second order curve from the N points (Xi, Yi)(where i=1, 2, . . . , N) collected in the state 212. The second-ordercurve may be defined by Y=A*X²+B*X+C (with i=1, 2, . . . , N) which goesthrough N points (Xi, Yi) so that the sum of (Y−Yi)² is minimal.

The state 216 may find the optimal value of the signal OFFSET from thesecond order curve Y=A*X²+B*X+C. The optimal value of the signal OFFSETmay be found when the signal OFFSET allows the peak-to-peak value Y ofthe signal DS to become maximal. The optimal value of the signal OFFSETmay be applied to signal FE via the adder 112 to obtain the best (orincreased) focus point for the laser beam 124 to read data on the disc104.

FIG. 9 illustrates the peak-to-peak value of the signal DS as the signalOFFSET is changed. FIG. 9 illustrates a measurement plot 220 and acurved plot 222. The optimal value of the signal OFFSET is shown as apoint 224. FIG. 9 illustrates that the point 224 may be at the secondvalue which is slightly off of the first value (or zero). The point 224may correspond to the optimal focus point of the laser beam 124 on thedisc 104.

Referring to FIG. 10, a method 250 for optimizing the focus point torecord data on an optical disc is shown. The method 250 generallycomprises a state (or step) 252, a state (or step) 254, a state (orstep) 256, a state (or step) 258, a state (or step) 260, a state (orstep) 262, a state (or step) 264, a state (or step) 266, and a state (orstep) 268. The state 252 may be a start state.

The state 254 may control the lens 126 to keep the laser beam 124focused on the surface of the disc 104. When the lens 126 is controlledso that the laser beam 124 is focused on the disc 104, the laser beam124 may not be controlled to lock on a particular one of a number ofphysical tracks 105 a-105 n of the disc 104. The state 256 may apply apredetermined amount of constant offset in the signal OFFSET. Thepredetermined amount of offset in the signal OFFSET may be added to thesignal FE to generate the signal FE_AFTER_OFFSET.

The state 258 may measure the average peak-to-peak value of the signalMBPP based on revolutions (e.g., R) of the spinning disc 104. The numberof revolutions R used by the method 250 to measure the averagepeak-to-peak value of the signal MBPP may be varied to meet the designcriteria of a particular implementation.

The state 260 may gradually adjust (by increasing or decreasing) thesignal OFFSET by a fixed predetermined amount. The state 262 may repeatstates 256, 258 and 260 a number of times (e.g., N). The step 258 maycollect Xi and Yi values (where i=1, 2, . . . , N). The Yi value maycorrespond to the measured peak-to-peak values of the signal MBPP whenthe states 256-258 are repeated N-times. The Xi value may correspond tothe adjusted values of the signal OFFSET when the state 260 is repeatedN-times.

The state 264 may find a second order curve from the N points (Xi, Yi)(where i=1, 2, . . . , N) collected in the state 260. The second-ordercurve may be defined by Y=A*X²+B*X+C (with i=1, 2, . . . , N) which goesthrough N points (Xi, Yi) so that the sum of (Y−Yi)² is minimal.

The state 266 may find the optimal value of the signal OFFSET from thesecond order curve Y=A*X²+B*X+C. The optimal value of the signal OFFSETmay be found when the signal OFFSET allows the peak-to-peak value Y ofthe signal MBPP to become maximal. The optimal value of the signalOFFSET may be applied to the signal FE via the adder 112 to obtain thebest (or increased) focus point for the laser beam 124 to record data onthe disc 104.

FIG. 11 illustrates the peak-to-peak value of the signal MBPP as thesignal OFFSET is changed. FIG. 11 illustrates a measurement plot 270 anda curved plot 272. The optimal value of the signal OFFSET is shown as apoint 274. FIG. 11 illustrates that the point 274 may be at a secondvalue which is slightly off of the first value (or zero). The point 274may correspond to the optimal focus point of the laser beam 124 torecord data on the disc 104.

The present invention may (i) optimize the focus point of the laser beam124 on the optical disc 104 based on whether (a) data is being read fromthe disc 104 or (b) data is being written to the disc 104, (ii) providea high degree of reliability by using a curved-fit technique to find theoptimal value of a focus offset, (iii) be easily implemented withinhardware and/or firmware or completely with firmware, and (iv) ensure ahigh degree of quality in optimizing the focus point of the laser beam124 to either read or record data on the optical disc 104.

The function performed by the flow diagram of FIGS. 8 and 10 may beimplemented using a conventional general purpose digital computerprogrammed according to the teachings of the present specification, aswill be apparent to those skilled in the relevant art(s). Appropriatesoftware coding can readily be prepared by skilled programmers based onthe teachings of the present disclosure, as will also be apparent tothose skilled in the relevant art(s).

The present invention may also be implemented by the preparation ofASICs, FPGAs, or by interconnecting an appropriate network ofconventional component circuits, as is described herein, modificationsof which will be readily apparent to those skilled in the art(s).

The present invention thus may also include a computer product which maybe a storage medium including instructions which can be used to programa computer to perform a process in accordance with the presentinvention. The storage medium can include, but is not limited to, anytype of disk including floppy disk, optical disk, CD-ROM,magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, Flash memory,magnetic or optical cards, or any type of media suitable for storingelectronic instructions. The present invention may be particularlyuseful in an optical disc system (e.g., CD type, DVD type, etc.). Thepresent invention may be useful in newly developing formats such asBlue-ray and HD-DVD systems.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the scope of the invention.

1. An apparatus comprising: a first circuit configured to generatestatus signals and an error signal in response to accessing a storagemedium, wherein said error signal provides a first value based on anaccuracy of accessing said storage medium; and a second circuitconfigured to offset said first value of said error signal to a secondvalue to increase the accuracy of accessing said storage medium, wherein(i) said status signals comprise one or more of a data signal and adifferential signal, (ii) in a first mode, an offset signal is generatedin response to said data signal, and (iii) in a second mode the offsetsignal is generated in response to said differential signal.
 2. Theapparatus according to claim 1, wherein said second circuit isconfigured to offset said first value of said error signal to saidsecond value to increase the quality of a reading process when readingdata from said storage medium.
 3. The apparatus according to claim 1,wherein said second circuit is configured to offset said first value ofsaid error signal to said second value to increase the quality of awriting process when writing data to said storage medium.
 4. Theapparatus according to claim 1, wherein said data signal is generatedwhen reading data from an optical disc and said differential signalcomprises a beam push-pull signal generated when writing or rewritingdata to said optical disc.
 5. The apparatus according to claim 1,wherein said data signal comprises a radio frequency signal.
 6. Theapparatus according to claim 1, wherein said first circuit comprises: adata circuit configured to generate said status signals in response to alaser beam focused on a disc; and an error circuit configured togenerate said error signal, wherein said error signal provides saidfirst value based on a focus point of said laser beam.
 7. The apparatusaccording to claim 6, further comprising: a sensor configured togenerate a plurality of sensory signals in response to said laser beam,wherein said data circuit generates said status signals in response tosaid plurality of sensory signals.
 8. The apparatus according to claim6, further comprising: a focus controller configured to control a lensin relation to said laser beam and allow said laser beam to focus onsaid disc.
 9. An apparatus comprising: means for generating a pluralityof status signals and an error signal, wherein said error signalprovides a first value based on an accuracy of an access of a storagemedium; and means for offsetting said first value of said error signalto a second value to increase the accuracy of accessing said storagemedium, wherein (i) said status signals comprise one or more of a datasignal and a differential signal, (ii) in a first mode, an offset signalis generated in response to said data signal, and (iii) in a second modethe offset signal is generated in response to said differential signal.10. A method for increasing accuracy of accesses to a storage medium,comprising the steps of: (A) generating status signals in response toaccessing said storage medium; (B) generating an error signal whichprovides a first value based on an accuracy of an access of said storagemedium; and (C) offsetting said first value of said error signal with anoffset signal to a second value to increase the accuracy of accessingsaid storage medium, wherein (i) said status signals comprise one ormore of a data signal and a differential signal, (ii) in a first mode,the offset signal is generated in response to said data signal, and(iii) in a second mode the offset signal is generated in response tosaid differential signal.
 11. The method according to claim 10, whereinstep (C) further comprises the step of: offsetting said first value ofsaid error signal said second value to increase the quality of a readingprocess when reading data from said storage medium; and offsetting saidfirst value of said error signal to said second value to increase thequality of a writing process when writing or rewriting data to saidstorage medium.
 12. The method according to claim 10, wherein saidstorage medium comprises a disc and said method further comprises thesteps of: controlling a lens in relation to a laser beam to allow saidlaser beam to focus on said disc; generating said status signals inresponse to said laser beam focused on said disc; and generating saiderror signal such that said first value is based on a focus point ofsaid laser beam.
 13. The method according to claim 12, furthercomprising the step of: controlling said lens in relation to said laserbeam to allow said laser beam to focus on said disc while at the sametime controlling said laser beam to lock on a physical track on saiddisc.
 14. The method according to claim 12, wherein step (C) furthercomprises the steps of: (D) applying a predetermined amount of constantoffset to said error signal with said offset signal; (E) measuring apeak-to-peak value of said data signal in response to applying saidpredetermined amount of constant offset; and (F) adjusting said offsetsignal by a fixed predetermined amount.
 15. The method of claim 14,further comprising the steps of: repeating steps (D), (E) and (F) anumber of times to collect a number of (i) peak-to-peak values of saiddata signal and (ii) adjusted values of said offset signal.
 16. Themethod according to claim 15, further comprising the steps of: finding asecond-order curve using (i) said number of peak-to-peak values of oneof said data signal and (ii) said adjusted values of the offset signal;and finding said second value from said second order curve.
 17. Themethod according to claim 16, further comprising the step of:controlling said lens in relation to said laser beam to allow said laserbeam to focus on said disc without controlling said laser beam to lockon a physical track on said disc.
 18. The method according to claim 17,wherein step (C) further comprises the steps of: (G) applying apredetermined amount of constant offset to said error signal with saidoffset signal; (H) measuring a peak-to-peak value of said differentialsignal in revolutions in response to applying said predetermined amountof constant offset; and (I) adjusting said offset signal by a fixedpredetermined amount.
 19. The method of claim 18, further comprising thesteps of: repeating steps (G), (H) and (I) a number of times to collecta number of (i) peak-to-peak values of said differential signal and (ii)adjusted values of said offset signal.
 20. The method according to claim19, further comprising the steps of: finding a second-order curve using(i) said number of peak-to-peak values of said differential signal and(ii) said adjusted values of the offset signal; and finding said secondvalue from said second order curve.